1. Field of the Invention
The present invention relates to an exposure apparatus comprising a chamber where an optical element having a capping layer is arranged, a control method for the exposure apparatus, and a device manufacturing method.
2. Description of the Related Art
Conventionally, as a lithography method to manufacture fine semiconductor devices, such as semiconductor memories and logic circuits, reduction projection exposure using ultraviolet rays has been performed. However, as the integration density of the semiconductor devices increases, the development of a technique has become a matter of great urgency, which can obtain a resolution with a minimum line width of 70 nm or less, which conventional photolithography arrangements employing ultraviolet light cannot achieve.
Recently, to transfer a very fine circuit pattern onto a photoresist, a photolithography technique that employs EUV (Extreme Ultraviolet) light having a shorter wavelength of 11 nm to 15 nm, to replace ultraviolet rays, has been developed. The EUV lithography technique is expected to be promising as a technique that can obtain a resolution with a minimum line width of 70 nm or less.
In the EUV region, which covers a wavelength of 11 nm to 15 nm, all substances have strong absorption. In this region, an optical system including a transmissive optical element, which utilizes refraction as in lithography employing ultraviolet light as the exposure light, cannot be employed. Hence, an optical system comprising a reflective optical element, such as a thin film filter or mirror, is employed. The surface of such a reflective optical element has a multi-layer film obtained by alternately stacking two types of substances having different optical constants. For example, alternate stacking of molybdenum (Mo) and silicon (Si) on the surface of a glass substrate polished into an accurate shape can form the multi-layer film. Regarding the thicknesses of the respective layers, for example, each Mo layer has a thickness of about 3 nm, and each Si layer has a thickness of about 4 nm.
A gas component present in the atmosphere also absorbs light within the EUV region, which covers a wavelength of 11 nm to 15 nm, to attenuate the light greatly. Thus, the interior of the exposure apparatus is maintained to such a vacuum degree that exposure light will not attenuate. Gases mainly containing water and a carbon-based substance remain in the vacuum atmosphere in the exposure apparatus. The residual gases include a gas generated by a member, such as a cable used in the exposure apparatus, and a gas volatilizing from a resist applied on a wafer.
The residual gas components repeat physical adsorption in the surface of the optical element used in the exposure apparatus and desorption from it. The time duration of adsorption in the optical element surface varies depending on the substances, and ranges from a minimum of several tens of picoseconds to a maximum of several thousand seconds. Usually, the residual gas components merely adsorb physically, and neither chemically combine with the optical element surface nor cause a reaction.
When, however, EUV light irradiates the optical element, secondary electrons are generated on the optical element surface to dissociate the residual gas components that have adsorbed in the optical element surface. Particularly, when water has been physically adsorbed, active substances, such as oxygen radicals or hydroxide radicals generated by the dissociation, react on the optical element surface to undesirably oxidize it.
When the optical element surface oxidizes, it degrades the performance of the optical element, to decrease the throughput. Particularly, in an EUV exposure apparatus, if the optical element is a reflective multi-layer mirror, oxidation of the uppermost layer by merely several nm leads to a decrease in reflectance. In the EUV exposure apparatus, even a slight decrease in reflectance of each multi-layer mirror may adversely affect the throughput of the exposure apparatus very largely. If surface oxidation occurs locally, it causes nonuniform illuminance to degrade the image performance of the exposure apparatus.
To prevent the surface oxidation, a method of forming a capping layer, which covers the surface of the multi-layer mirror, has been proposed. As the material of the capping layer, a more inactive material should be selected. For example, groups including diamond-like carbon, boron nitride, boron carbide, silicon nitride, silicon carbide, B, Pd, Ru, Rh, Au, MgF2, LiF, C2F4, and TiN, and their compounds and alloys are useful (see, for example, Japanese Patent Laid Open No. 2001-59901 that corresponds to U.S. Pat. No. 6,449,086 B1). By using such a resistant material to form the capping layer, the oxidation resistance can improve.
Formation of a capping layer on the optical layer in order to prevent surface oxidation is an effective way to prevent degradation of the optical element so as to prolong the service life of the optical element. It is, however, very difficult to prevent surface oxidation of the optical element semipermanently, to maintain the performance of the exposure apparatus.
Once the optical element oxidizes, it cannot be restored. Hence, to prolong the service life of the optical element, it is indispensable to avoid oxidation.
Particularly, when the capping layer is made of a substance which decreases by oxidation, oxidation of an underlying layer is also likely to progress. For example, when the capping layer is a carbon film, oxidized carbon forms carbon dioxide, carbon monoxide, or the like, to decrease the carbon content of the film. When the carbon film becomes thin or less dense, not only do the optical characteristics change due to the carbon film, but also, oxidation of the underlying layer is likely to progress.